Synthesis and Crystal Structures of [(dmba)Pd{Ph - American

Université Henri Poincaré Nancy 1, B.P. 239, F-54506 Vandoeuvre-les-Nancy Cédex, France. ReceiVed NoVember 22, 1995X. Electrophilic attack of 1 equ...
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7174

Inorg. Chem. 1996, 35, 7174-7180

Iodination of r-Phosphino Enolate Complexes of Palladium(II) and Platinum(II). Synthesis and Crystal Structures of [(dmba)Pd{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] and of the Dipalladium(II) Complex [(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)(tmeda)] Obtained by Palladium(0) Insertion into the Carbon-Iodine Bond Jacques Andrieu,† Pierre Braunstein,*,† Yves Dusausoy,‡ and Nour Edine Ghermani‡ Laboratoire de Chimie de Coordination, Associe´ au CNRS (URA 416), Universite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070 Strasbourg Ce´dex, France, and Laboratoire de Cristallographie et Mode´lisation des Mate´riaux Mine´raux et Biologiques, Associe´ au CNRS (URA 809), Universite´ Henri Poincare´ Nancy 1, B.P. 239, F-54506 Vandoeuvre-les-Nancy Ce´dex, France ReceiVed NoVember 22, 1995X

Electrophilic attack of 1 equiv of I2 on a PCsp2 carbon of the Pt(II) complex cis-[Pt{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}2] (1) afforded cis-[Pt(I)[Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}{Ph2PCH(I)C(O)Ph}] (2) in 90% yield. Complex 2 was subsequently deprotonated by NaOEt in ethanol to give the bis(enolato) complex cis-[Pt{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}{Ph2PC (I) ‚ ‚ C( ‚ ‚ O)Ph}] (3). This R-phosphino, R-iodo enolato complex was obtained directly and quantitatively by the reaction of 1 with 1 equiv of N-iodosuccinimide (NIS). When 2 equiv of NIS was used, the symmetrical complex cis-[Pt{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}2] (4) was formed selectively. In contrast to I2, NIS was also able to functionalize the phosphino enolate ligand of complexes [(C N)Pd{Ph2PCH ‚ ‚C( ‚ ‚O)Ph}] to give the corresponding iodo derivatives [(C N)Pd{Ph2PC(I) ‚ ‚C( ‚ ‚O)Ph}] (C N ) dmba (5) or 8-mq (6)). These represent the first examples in which a phosphino enolate C-H bond has been directly functionalized, i.e. replaced by a C-X bond.

Attempts to use this procedure with cis-[Ni{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}2] or with [Ni(Ph)

{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}(PPh3)] were unsuccessful. Reaction of 5 with Pd(dba)2 in the presence of tetramethylenediamine (tmeda) or 2,2′-bipyridine (bipy) afforded [(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)(tmeda)] (7) and [(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)(bipy)] (8), respectively. The solid state structures of complexes 5 and 7‚CH2Cl2 have been determined by single-crystal X-ray diffraction: 5 crystallizes in the monoclinic space group P21/n with Z ) 4 in a unit cell of dimensions a ) 12.867(3) Å, b ) 10.625(3) Å, c ) 19.509(6) Å, and β ) 102.23(2)°; 7‚CH2Cl2 crystallizes in the monoclinic space group C2/c with Z ) 8 in a unit cell of dimensions a ) 35.906(3) Å, b ) 13.565(3) Å, c ) 15.775(2) Å, and β ) 95.099(10)°. Complex 7 contains two palladium(II) centers, in a square-planar environment, connected by the P-C unit of a phosphino enolate ligand which adopts an unprecedented µ-η2(P,C):η2(P,O) bonding mode. The two coordination planes are almost orthogonal and make a dihedral angle of 88.0(2)°, which minimizes the steric hindrance between the ligands.

Introduction In the course of our studies on the synthesis and reactivity of mononuclear phosphino enolate complexes of the transition metals, we considered the possibility of using them as precursors for the synthesis of heterometallic, functional complexes. Air-stable, square planar complexes of the type [(C N)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)R}] (C N ) cyclometalated dimethylbenzylamine or 8-methylquinoline; R ) Ph, OEt, NPh2) and cis-[M{Ph2PCH ‚ ‚ C( ‚ ‚ O)R}2] (M ) Ni, Pd, Pt) are excellent candidates for the study of chemoselective reactions

between the enolate moiety and electrophilic reagents, and we have conducted such studies with electrophiles like carbon dioxide, organic nitriles and isocyanates, activated alkynes and chlorophosphines.1 However, little is known about the reactivity of these phosphino enolate complexes with electrophilic metal centers. We have found that complexes of the softer metal ions Pd(II) and Au(I) reacted with formation of a PCsp2-metal bond, as in [(C N)Pd(µ-Cl){µ-Ph2PCHC(O)OEt}Pd(C N)] (eq 1)2 or in [(C N)Pd{Ph2PCH(AuPPh3)C(O)Ph}](BF4) (eq 2).3 In contrast, the “harder” Co(II) center in CoI2 is chelated by the oxygen atoms of the nickel complex cis-[Ni{Ph2PCH ‚ ‚ C

†

Universite´ Louis Pasteur. ‡ Universite ´ Henri Poincare´ Nancy. X Abstract published in AdVance ACS Abstracts, October 15, 1996.

S0020-1669(95)01507-2 CCC: $12.00

( ‚ ‚ O)Ph}2] to give a heterometallic, paramagnetic molecule constituted by two metal complexes (eq 3).3b © 1996 American Chemical Society

Iodination of R-Phosphino Enolates

Inorganic Chemistry, Vol. 35, No. 24, 1996 7175 Scheme 1

the way to interesting new reactivity. Intermediates containing a Pd-Csp2 bond are involved in numerous transformations, such as Heck-type carbon-carbon coupling reactions with alkynes or alkenes4 and the synthesis of mixed-valence ruthenium complexes5 or of organometallic polymers with interesting properties in nonlinear optics.6 Thus, the question was how to replace the enolato C-H bond with a C-metal bond, and we decided to explore the strategy depicted in Scheme 1. It involves first formation of a carbon-halogen bond that could allow subsequent insertion of a zerovalent metal fragment. This approach does not appear to have been much investigated in the past. Results Reaction

of

the

Pt(II)

complex

cis-[Pt{Ph2PCH ‚ ‚

C( ‚ ‚ O)Ph}2] (1)1f with 1 equiv of iodine in CH2Cl2 afforded cis-[Pt(I){Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}{Ph2PCH(I)C(O)Ph}] (2) in 90% yield (eq 4). The IR spectrum of 2 shows typical

Since the enolato PC-H bond is retained in all these reactions, formation of a bond between the PCsp2 carbon and another carbon or a metal results in the generation of a PCsp3 center. We wondered whether our systems could lead to stable heterometallic metallo enolate complexes, i.e. complexes in which the second metal would be covalently linked to the enolato sp2 carbon of the precursor molecule. This could open (1) (a) Braunstein, P.; Matt, D.; Dusausoy, Y.; Protas, J. J. Chem. Soc., Chem. Commun. 1979, 763. (b) Braunstein, P.; Matt, D.; Dusausoy, Y.; Fischer, J.; Mitschler, A.; Ricard, L. J. Am. Chem. Soc. 1981, 103, 5115. (c) Braunstein, P.; Matt, D.; Dusausoy, Y.; Fischer, J. Organometallics 1983, 2, 1410. (d) Braunstein, P.; Matt, D.; Nobel, D. Chem. ReV. 1988, 88, 747. (e) Braunstein, P.; Matt, D.; Nobel, D.; Bouaoud, S.-E.; Grandjean, D. J. Organomet. Chem. 1986, 301, 401. (f) Bouaoud, S.-E.; Braunstein, P.; Grandjean, D.; Matt, D.; Nobel, D. Inorg. Chem. 1986, 25, 3765. (g) Bouaoud, S.-E.; Braunstein, P.; Grandjean, D.; Matt, D.; Nobel, D. J. Chem. Soc., Chem. Commun. 1987, 488. (h) Braunstein, P.; Matt, D.; Nobel, D.; Fischer, J. J. Chem. Soc., Chem. Commun. 1987, 1530. (i) Braunstein, P.; Matt, D.; Nobel, D. J. Am. Chem. Soc. 1988, 110, 3207. (j) Bouaoud, S.-E.; Braunstein, P.; Grandjean, D.; Matt, D.; Nobel, D. Inorg. Chem. 1988, 27, 2279. (k) Braunstein, P.; Matt, D.; Nobel, D.; Balegroune, F.; Bouaoud, S.-E.; Grandjean, D.; Fischer, J. J. Chem. Soc., Dalton Trans. 1988, 353. (l) Balegroune, F.; Braunstein, P.; Grandjean, D.; Matt, D.; Nobel, D. Inorg. Chem. 1988, 27, 3320. (m) Braunstein, P.; Nobel, D. Chem. ReV. 1989, 89, 1927. (n) Balegroune, F.; Braunstein, P.; Gomes Carneiro, T. M.; Grandjean, D.; Matt, D. J. Chem. Soc., Chem. Commun. 1989, 582. (o) Braunstein, P.; Gomes Carneiro, T. M.; Matt, D.; Balegroune, F.; Grandjean, D. Organometallics 1989, 8, 1737. (2) Braunstein, P.; Matt, D.; Fischer, J.; Ricard, L.; Mitschler, A. New J. Chem. 1980, 4, 493. (3) (a) Veya, P.; Floriani, C.; Chiesi-Villa, A.; Guastini, C.; Dedieu, A.; Ingold, F.; Braunstein, P. Organometallics 1993, 12, 4359. (b) Andrieu, J.; Braunstein, P.; Drillon, M.; Dusausoy, Y.; Ingold, F.; Rabu, P.; Tiripicchio, A.; Ugozzoli, F. Inorg. Chem., in press.

absorptions at 1507 and 1676 cm-1 for the enolate and the keto functions, respectively. The resonances for the P atoms appear at δ 39.1 and -8.4 in the 31P{1H} NMR spectrum and show a 2J(PP) coupling of 10 Hz, typical for a cis arrangement. An electrophilic attack of I2 on the PCsp2 carbon, followed by coordination of the iodide anion to platinum accounts for the formation of 2. With the corresponding palladium and nickel complexes, a similar reaction led respectively to decomposition or formation of a red, paramagnetic compound that was not further investigated. In the reaction of Li[Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph] with I2, carried out for comparison, no product could be characterized although iodine oxydation reactions of Li[Ph2PCHPPh2] and Li[Ph2PNPPh2] have recently led to interesting P-P, P-C, C-C, and P-N coupling products.7 Obviously, the nature of the cation associated with the phosphino enolate ligand strongly influences its reactivity. Complex 2 was subsequently deprotonated by NaOEt in ethanol to give cis-[Pt{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] (3) (eq 5). Its IR spectrum contains absorptions at 1521 and 1506 cm-1 which correspond to the two different (4) (a) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic Press: London, 1985. (b) de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379. (c) DeVries, R. A.; Mendoza, A. Organometallics 1994, 13, 2405. (5) Kasahara, Y.; Hoshino, Y.; Kajitani, M.; Shimizu, K.; Satoˆ, G. P. Organometallics 1992, 11, 1968. (6) Ingham, S. L.; Khan, M. S.; Lewis, J.; Long, N. J.; Raithby, P. R. J. Organomet. Chem. 1994, 470, 153. (7) (a) Braunstein, P.; Hasselbring, R.; Tiripicchio, A.; Ugozzoli, F. J. Chem. Soc., Chem. Commun. 1995, 37. (b) Braunstein, P.; Hasselbring, R.; Tiripicchio, A.; Ugozzoli, F. Bull. Soc. Chim. Fr. 1995, 132, 691.

7176 Inorganic Chemistry, Vol. 35, No. 24, 1996

Andrieu et al. Scheme 2

phosphino enolate ligands. The 31P{1H} NMR spectrum contains two doublets at δ 27.1 and 13.8 which show a characteristic 2J(PP) cis coupling of 11.8 Hz. Interestingly, complex 3 could be obtained directly and quantitatively by the reaction of 1 with 1 equiv of N-iodosuccinimide (NIS). When 2 equiv of NIS was used, the symmetrical complex 4 was formed selectively (Scheme 2). Its 31P{1H} NMR spectrum contains a singlet at δ 28.0 with a 1J(Pt-P) coupling constant of 3542 Hz, in agreement with a cis structure (Scheme 2). The 1H NMR spectrum of 4 indicates the loss of the PCH proton. The ν(C ‚ ‚ O) + ν(C ‚ ‚ C) absorption of 1 at 1517 cm-1 is shifted to 1505 cm-1 in 4, a value similar to one of the two absorptions found in 3. In contrast to I2, NIS is able to functionalize the phosphino

step in the Heck reaction.4 Indeed, 5 did not react with Pd(PPh3)4 in toluene at 60 °C. However, the desired reaction took place when nitrogen-containing ligands were coordinated to palladium(0). Thus, reaction of 5 with Pd(dba)2 in the presence of tetramethylenediamine (tmeda) or 2,2′-bipyridine (bipy) afforded [(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)(tmeda)] (7) and [(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)(bipy)] (8), respectively (eq 7). The 1H NMR spectrum of 7 and 8 shows an ABX-

enolate ligand of complexes [(C N)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O) Ph}] to give the iodo derivatives [(dmba)Pd{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] (5) and [(8-mq)Pd{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] (6) (eq 6). The succinimide formed is easily removed with water

which does not protonate the enolate carbon atom of complexes 5 or 6. In the IR spectrum, the ν(C ‚ ‚ C) + ν(C ‚ ‚ O) absorption of 5 and 6 is found at 1495 and 1501 cm-1, respectively. In order to analyze the structural influence of the iodo substituent, an X-ray diffraction study was performed on 5 (see below). Attempts to extend this procedure to cis-[Ni{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}2]1k or [Ni(Ph){Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}(PPh3)], the classical SHOP-type catalyst model,8 were unsuccessful. Broad signals were observed in the 1H and 31P {1H} NMR spectra which suggested the presence of paramagnetic complexes. With the aim of exploiting the newly created C-I functionality of 5 or 6 for further coupling reactions, we reacted 5 with HCtCPh or HCtCSiMe3 in the presence of Pd(PPh3)4 and CuI.5,9 Since no reaction was observed, we wondered whether this was due to a lack of reactivity of the C-I bond toward Pd(0). Oxidative insertion of the metal catalyst species into the aryl-halogen bond generally represents the rate-determining (8) (a) Keim, W. New J. Chem. 1987, 11, 531. (b) Klabunde, U.; Tulip, T. H.; Roe, D. C.; Ittel, S. D. J. Organomet. Chem. 1987, 334, 141. (c) Hirose, K.; Keim, W. J. Mol. Catal. 1992, 73, 271. (d) Braunstein, P.; Chauvin, Y.; Mercier, S.; Saussine, L.; DeCian, A.; Fischer, J. J. Chem. Soc., Chem. Commun. 1994, 2203. (9) (a) de Graaf, W.; van Wegen, J.; Boersma, J.; Spek, A. L.; van Koten, G. Recl. TraV. Chim. Pays-Bas 1989, 108, 275. (b) Markies, B. A.; Canty, A. J.; de Graaf, W.; Boersma, J.; Janssen, M. D.; Hogerheide, M. P.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. J. Organomet. Chem. 1994, 482, 191. (c) J. Vicente, J.-A. Abad, R. Bergs, P. G. Jones, D. Bautista, J. Chem. Soc., Dalton Trans. 1995, 3093.

type pattern (X ) P) for the dmba methylene protons, consistent with the lack of any symmetry element in these molecules. As expected, the IR spectrum contains no absorption due to a coordinated ketone (which would have resulted if the enolate carbon had become sp3-hybridized) but the enolate absorption was obscured by other strong ligand absorptions around 1490 cm-1. In order to firmly establish the molecular structure of this unusual dinuclear complex, an X-ray diffraction study was performed on a single crystal of 7 (see below). Whereas the tmeda ligand in cis-[Pd(Ph)I(tmeda)] is easily displaced by chelating nitrogen or phosphorus donors,9a,b this is not the case with 7. Thus, no reaction was observed between 7 and 1 equiv of bipy (CH2Cl2, 3 h) although the expected product 8 formed directly in the reaction of eq 7. Similarly, 7 did not react with dppe (toluene, 60 °C, 6 h). Obviously, the kinetic inertness of 7 in these reactions must be due to steric factors. The crystal structure determination of 7 (see below) showed that the coordination planes of the two palladium atoms are almost orthogonal (88.0(2)°) and that the phenyl rings C(11)-C(16) at P and C(25)-C(30) at C(24) might generate steric shielding preventing easy attack of a nucleophile at Pd(1) to generate a five-coordinate transition state. When complex 7 was reacted with the PhCtCH/NEt3/CuI system,10a only [(dmba)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}]1f could be isolated. The hydrogen atom that replaced the palladium σ-bonded to the PCsp2 carbon of 7 originates from the am(10) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 50, 4467. (b) Cassar, L. J. Organomet. Chem. 1975, 93, 253. (c) Dieck, H. A.; Heck, F. R. J. Organomet. Chem. 1975, 93, 259.

Iodination of R-Phosphino Enolates

Inorganic Chemistry, Vol. 35, No. 24, 1996 7177 Table 1. Selected Bond Distances (Å) and Angles (deg) for 5

Figure 1. ORTEP view of the molecular structure of [(dmba)Pd {Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] (5).

monium salt [HNEt3]I which is formed. This was independently confirmed by the reaction of 7 with 1 equiv of [HNEt3]Cl, which yielded [(dmba)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}]. The reactivity of the Pd-Csp2 bond of 7 therefore requires the use of aprotic reagents. We also tried to perform C-C coupling reactions in basic media, as described by Cassar or Heck.10b,c No reaction was observed between 7 and the sodium phenylacetylide generated in situ from PhCtCH and NaOMe. This lack of reactivity may be ascribed to the steric factors discussed above. Crystal Structures of [(dmba)Pd{Ph2PC(I) ‚ ‚ C( ‚ ‚ O) Ph}]

(5)

and

[(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)

(tmeda)]‚CH2Cl2 (7‚CH2Cl2). A view of the molecular structure of complex 5 is shown in Figure 1; selected bond distances and angles are given in Table 1. The palladium atom has a square planar environment involving the N and C(21) atoms of the three-electron donor cyclometalated ligand and P and O atoms of the three-electron donor phosphino enolate ligand. The oxygen atom of one five-membered chelate ring is trans with respect to the C(21) atom of the other ring. The maximum deviations from the mean plane containing the Pd, P, C(1), C(2), and O atoms are found for palladium (-0.0518(4) Å) and oxygen (0.058(4) Å) and the iodine atom is at a distance of 0.0423(4) Å. The carbon atoms C(1) and C(2) are situated on the same side of the palladium coordination plane, at 0.24(1) and 0.17(2) Å. The dmba chelate has an envelope conformation with C(27) being -0.481(5) Å out of the mean plane passing through the Pd, C(21), C(26), and N atoms. The bond distances to Pd are in the normal range. The dimensions involving the dmba chelate are similar to those determined for this ligand in other Pd(II) complexes.1e,11 The Pd-O distance of 2.074(3) Å is only slightly shorter than that in [(dmba)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)OEt}] (2.117(5) Å)1b or in (11) Canty, A. J. In ComprehensiVe Organometallic Chemistry, 2nd ed.; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, England, 1995; Vol. 9, Chapter 5, pp 1-72.

Pd-P Pd-O Pd-C21 Pd-N I-C1 P-C1 P-C3 P-C9 C1-C2 C2-O C2-C15

2.238(1) 2.074(3) 1.986(4) 2.135(4) 2.099(5) 1.792(5) 1.810(4) 1.830(5) 1.359(6) 1.300(6) 1.489(6)

P-Pd-O P-Pd-C21 P-Pd-N O-Pd-C21 O-Pd-N C21-Pd-N Pd-P-C1 Pd-P-C3 Pd-P-C9 C1-P-C3 C1-P-C9 C3-P-C9 I-C1-P I-C1-C2 P-C1-C2 C1-C2-O

83.99(9) 101.0(1) 175.1(1) 174.9(2) 92.0(1) 83.0(2) 98.7(1) 117.6(2) 118.2(2) 109.7(2) 107.1(2) 104.9(2) 118.7(2) 124.1(3) 116.7(4) 121.6(4)

C21-C22 C21-C26 C22-C23 C23-C24 C24-C25 C25-C26 C26-C27 N-C27 N-C28 N-C29 Pd-C21-C22 Pd-C21-C26 C22-C21-C26 Pd-N-C27 Pd-N-C28 Pd-N-C29 C27-N-C28 C27-N-C29 C28-N-C29 C26-C27-N C1-C2-C15 O-C2-C15 Pd-O-C2 C21-C26-C25 C21-C26-C27 C25-C26-C27

1.341(6) 1.415(6) 1.402(7) 1.378(8) 1.364(8) 1.392(7) 1.481(7) 1.492(6) 1.473(7) 1.473(7) 131.0(3) 111.8(3) 117.2(4) 105.0(3) 113.0(3) 109.4(3) 110.6(4) 109.7(4) 109.1(4) 109.8(4) 125.5(4) 112.9(4) 118.3(3) 120.1(4) 118.3(4) 121.6(4)

a Numbers in parentheses are estimated standard deviations in the least significant digits.

[(dmba)Pd{Ph2PCH2C(O)O}] (2.105(3) Å).1e The C(1)-I distance of 2.099(5) Å corresponds to a Csp2-I bond and to the sum of the covalent radii of sp2-carbon (0.74 Å) and iodine (1.33 Å). The C(1)-C(2) and C(2)-O distances of 1.359(6) Å and 1.300(6) Å are fully consistent with their partial double bond character and electron delocalization within the P,O chelate. The crystal structure of 7‚CH2Cl2 consists of discrete molecular units separated by normal van der Waals contacts and dichloromethane molecules of solvation (Figure 2). Selected bond lengths and angles are listed in Table 2. The molecule contains two palladium(II) centers, in square-planar environments, connected by the P-C unit of a phosphino enolate ligand which adopts an unprecedented µ-η2(P,C): η2(P,O) bonding mode. The two coordination planes are almost orthogonal and make a dihedral angle of 88.0(2)°, which minimizes the steric hindrance between the ligands. The coordination around Pd(2) is very similar to that of the palladium atom in 5 and does not need therefore to be discussed in detail. The atoms C(2) and C(3) are situated on the same side of the Pd(2) coordination plane, at 0.61(1) and 0.35(1) Å, respectively. The slight deformation of the Pd(2) coordination plane manifests itself by the distances of Pd(2) (0.045(2) Å), P (-0.097(3) Å), O (0.082(3) Å), N(1) (-0.107(3) Å), and C(4) (0.076(3) Å) to the mean plane passing through these atoms. The coordination around Pd(1) is defined by the two nitrogen donors of the chelating tmeda ligand, an iodide ligand and the sp2-hybridized carbon atom C(23) of the phosphino enolate ligand which chelates Pd(2). This is similar to the situation in Pd(Ph)I(tmeda).9a,b The C(23)-Pd(1)-I angle of 89.3(2)° reflects the lack of steric constraint between these ligands. The difference of 0.053 Å between the Pd(1)-N(3) and Pd(1)-N(2) bond distances results from the higher trans-influence of the σ-bonded sp2 C(23) atom.9c The Pd(1)-C(23) and Pd(2)-C(4) distances are similar (2.019(7) and 1.997(8) Å, respectively) and the Pd(1)-I distance of 2.596(1) Å, which falls near the

7178 Inorganic Chemistry, Vol. 35, No. 24, 1996

Andrieu et al. Table 2. Selected Bond Lengths (Å) and angles (deg) for 7‚CH2Cl2 Pd(1)-C(23) Pd(1)-I Pd(1)-N(2) N(2)-C(31) N(2)-C(35) N(2)-C(36) Pd(1)-N(3) N(3)-C(32) N(3)-C(33) N(3)-C(34) Pd(2)-P P-C(23) P-C(11) C(23)-Pd(1)-N(2) C(23)-Pd(1)-N(3) N(2)-Pd(1)-N(3) C(23)-Pd(1)-I N(2)-Pd(1)-I N(3)-Pd(1)-I C(4)-Pd(2)-O C(4)-Pd(2)-N(1) O-Pd(2)-N(1) C(4)-Pd(2)-P O-Pd(2)-P N(1)-Pd(2)-P C(23)-P-Pd(2) C(2)-N(1)-Pd(2)

Figure 2. ORTEP view of the molecular structure of [(dmba)Pd {Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)(tmeda)] in 7‚CH2Cl2: (a) top view; (b) view showing the almost orthogonal orientation of the metal coordination planes.

lower quartile (2.593 Å) for this bond in the sample of 15 structures analyzed in 1989,12 is longer than that in Pd(Ph)I(tmeda) (2.5703(8) Å).9a,b Whereas the enolate C(23)-C(24) bond of 1.356(11) Å has the same length as the corresponding C(1)-C(2) bond in 5 (1.359(6) Å), consistent with a strong double bond character, the C(24)-O bond of 1.344(9) Å is slightly longer than that in 5 (1.300(6) Å). Discussion

2.019(7) 2.5963(9) 2.159(7) 1.459(12) 1.452(13) 1.427(12) 2.212(7) 1.475(13) 1.461(13) 1.45(2) 2.228(2) 1.795(8) 1.824(7) 94.4(3) 176.1(3) 82.8(3) 89.3(2) 175.4(2) 93.7(2) 174.1(3) 82.4(3) 92.2(2) 103.0(2) 82.6(2) 170.6(2) 102.5(3) 105.6(5)

P-C(17) Pd(2)-C(4) Pd(2)-N(1) N(1)-C(2) N(1)-C(9) N(1)-C(10) C(2)-C(3) C(3)-C(4) Pd(2)-O O-C(24) C(23)-C(24) C(24)-C(25) C(31)-C(32) C(31)-N(2)-Pd(1) C(32)-N(3)-Pd(1) C(24)-O-Pd(2) C(3)-C(2)-N(1) C(4)-C(3)-C(2) C(3)-C(4)-Pd(2) C(24)-C(23)-P C(24)-C(23)-Pd(1) P-C(23)-Pd(1) O-C(24)-C(23) C(23)-C(24)-C(25)

1.830(8) 1.997(8) 2.145(6) 1.494(11) 1.478(11) 1.470(11) 1.483(12) 1.418(11) 2.068(6) 1.344(9) 1.356(11) 1.507(11) 1.39(2) 104.0(6) 104.8(6) 117.2(5) 108.8(7) 118.7(8) 111.8(6) 111.6(6) 128.0(6) 120.3(4) 124.6(8) 125.3(8)

of NIS occurred selectively at the Csp2 carbon of the phosphino enolate ligand to give 5 and 6, respectively. These complexes are air-stable and are not hydrolized by H2O. The failure to observe a Pd/Cu-catalyzed coupling with terminal alkynes is not due to the inability of the C-I bond to react with the Pd(0) catalyst, although the nature of the ligands coordinated to Pd(0) plays a key role. This was shown by the reaction of eq 7 which required the presence of nitrogen donor ligands, such as tmeda or bipy, instead of phosphines. Complexes 7 and 8 were obtained under experimental conditions similar to those used to prepare cis-[Pd(Ph)I(tmeda)] from Pd(dba)2, tmeda, and Ph-I,9a,b indicating a similar reactivity of the C-I bond in 5 and in phenyl iodide. Note that by analogy with the protonation reaction which converts 7 into [(dmba)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}], the reaction of the isolobal cation [Au(PPh3)]+ with 5 (felt to be less reactive than 7 or 8 and therefore a better candidate) led to decomposition of the addition product. We confidently assign structure A to this intermediate by analogy with that of the

R-Phosphino, R-iodo enolate ligands are conveniently obtained by the reaction of palladium or platinum complexes containing the M[Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph] (M ) Pd, Pt) moiety with N-iodosuccinimide. These represent the first examples in which the phosphino enolate C-H bond has been directly functionalized, i.e. replaced by a C-X bond. With complexes containing two chelating phosphino enolate ligands, the reaction can be controled by stoichiometry which allows introduction of the C-I bond in only one or in both chelates (Scheme 2). Complex 3 was also prepared by the reaction of 2 with NaOEt in EtOH, the latter complex being obtained by iodine addition to 1. The formation of C-I and Pt-I bonds in the reaction of eq 4 illustrates the heterolytic I+/I- behavior of iodine. Reaction

of

the

palladium

complexes

[(C N)Pd

{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}] (C N ) dmba, 8-mq) with 1 equiv (12) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.

structurally characterized Pd-Au complex [(8-mq)Pd{Ph2PCH(AuPPh3)-C(O)Ph}](BF4).3b The geminal arrangement of the bulky iodide and Au(PPh3) substituents is obviously too destabilizing. In previous studies on phosphino enolate complexes of the d8 metals, which led to addition reactions at the enolate carbon (CO2, activated alkynes, organic nitriles and isocyanates, [Au(PPh3)]+, [Pd(µ-Cl)(C N)]2) or at the enolate oxygen (chlorophosphines, CoI2), the phosphino enolate ligand behaved as a 3- or 5-electron donor.1 (Note also the situation in the

Iodination of R-Phosphino Enolates

Inorganic Chemistry, Vol. 35, No. 24, 1996 7179

Scheme 3. Bonding Modes for the Phosphino Enolate P,O Ligand in Metal Complexes

Ru3clustershown below where this ligand behaves as a 5-electron donor µ-η1(O):η2(P,O) ligand13).

In the present work for the first time, the enolate C-H proton has been replaced by a functional group (iodine) which allows the synthesis of a new type of dinuclear complexes in which the phosphino enolate behaves as a 4-electron donor dianionic ligand. These various situations are schematically presented below (Scheme 3). Experimental Section A. Reagents and Physical Measurements. All reactions were performed in Schlenk-type flasks under nitrogen. Solvents were purified and dried under nitrogen by conventional methods. The 1H and 31P{1H} NMR spectra were recorded at 300.13 and 121.5 MHz, respectively, on a FT Bruker AC 300 instrument. IR spectra were recorded in the 4000-400 cm-1 range on a Bruker IFS66 FT spectrometer. B. Syntheses. The complexes Pd(dba)2 (dba ) dibenzylideneacetone),14 cis-[M{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}2] (M ) Ni, Pd, Pt) and [(C N)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}] (C N) ) dmba, 8-mq) were prepared according to procedures reported previously.1k cis-[Pt(I){Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}{Ph2PCH(I)C(O)Ph}] (2). A solution of I2 (0.215 g, 0.830 mmol) in CH2Cl2 (10 mL) was added dropwise to a solution of complex 1 (0.663 g, 0.827 mmol) in CH2Cl2 (20 mL). The yellow solution was filtered and concentrated to twothirds its original volume. Addition of pentane afforded yellow crystals of 2 (0.785 g, 90%), mp > 160 °C dec. IR (KBr): ν(CO) 1676 vs, ν(C ‚ ‚ C) + ν(C ‚ ‚ O) 1507 s. 1H NMR (CDCl3): δ 8.18-6.82 (m, 31 H, aromatic + P1CH(I)), 4.81 (d, with Pt satellites, 1 H, P2CH, 2J(PH) ) 10.7 Hz, 3J(PtH) ) 25.5 Hz). 31P{1H} NMR (CDCl ): δ 3 39.1 (d with Pt satellites, 1 P, 2J(PP) ) 10 Hz, 1J(PtP) ) 3454 Hz), -8.4 (d with Pt satellites, 1 P, 2J(PP) ) 10 Hz, 1J(PtP) ) 3591 Hz). Anal. Calcd for C40H32I2O2P2Pt (M ) 1055.53): C, 45.52; H, 3.06. Found: C, 45.27; H, 3.00. cis-[Pt{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] (3). Method 1. A solution of NaOEt (prepared from 0.20 g of Na) in EtOH (13) Braunstein, P.; Coco Cea, S.; Bruce, M. I.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 1992, 423, C38. (14) (a) Ito, T.; Takahashi, Y.; Ishii, Y. J. Chem. Soc., Chem. Commun. 1972, 629. (b) Moseley, K.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1974, 169.

(10 mL) was added to a solution of complex 2 (0.208 g, 0.224 mmol) in EtOH (20 mL). After being stirred for 1 h, the solution was filtered and the solvent was removed in Vacuo. The white residue was washed with EtOH (10 mL) and dried in Vacuo. Recrystallization from CH2Cl2/pentane afforded white needles of 3‚0.5CH2Cl2 (0.166 g, 80%), mp > 210 °C. IR (KBr): ν(C ‚ ‚ C) + ν(C ‚ ‚ O) 1521 s, 1506 s. 1H NMR (CDCl3): δ 7.96-7.03 (m, 30 H, aromatic), 4.80 (d with Pt satellites, 1 H, PCH, 2J(PH) ) 7.0 Hz, 3J(PtH) ) 30.7 Hz). 31P{1H} NMR (CDCl3): δ 27.1 (d with Pt satellites, 1 P, 2J(PP) ) 11.8 Hz, 1J(PtP) ) 3495 Hz), 13.8 (d with Pt satellites, 1 P, 2J(PP) ) 11.8 Hz, 1J(PtP) ) 3552 Hz). Anal. Calcd for C H IO P Pt‚0.5 CH Cl (M 40 31 2 2 2 2 ) 927.61 + 42.46): C, 50.14; H, 3.32. Found: C, 49.85; H, 3.45. Method 2. A solution of N-iodosuccinimide (0.055 g, 0.244 mmol) in THF (5 mL) was added to a solution of complex 1 (0.191 g, 0.238 mmol) in THF (20 mL). After being stirred for 1 h, the solvent was removed in Vacuo. The residue was washed several times with water (10 mL) in order to remove the succinimide formed and dried in Vacuo. Recrystallization from CH2Cl2/pentane afforded white needles of complex 3 (0.160 g, 70%). cis-[Pt{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}2] (4). A solution of N-iodosuccinimide (0.078 g, 0.354 mmol) in THF (5 mL) was added to a solution of complex 1 (0.131 g, 0.163 mmol) in THF (10 mL). After being stirred for 4 h, the solvent was removed in Vacuo. The residue was washed several times with water (10 mL). The product was recrystallized from THF/pentane and white crystals of 4 were obtained (0.110 g, 64%), mp > 180-190 °C dec. IR (KBr): ν(C ‚ ‚ C) + ν (C ‚ ‚ O) 1505 s. 1H NMR (CDCl3): δ 7.65-7.05 (m, 30 H, aromatic). 31P{1H} NMR (CH2Cl2/C6D6): δ 28.0 (s with Pt satellites, 1J(PtP) ) 3542 Hz). Anal. Calcd for C H I O P Pt (M ) 1053.52): 40 30 2 2 2 C, 45.60; H, 2.87. Found: C, 45.65; H, 2.87. [(dmba)Pd{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] (5). A solution of N-iodosuccinimide (0.208 g, 0.924 mmol) in THF (10 mL) was added to a solution of [(dmba)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}] (0.500 g, 0.919 mmol) in THF (20 mL). After being stirred for 2 h, the solvent was removed in Vacuo. The residue was washed several times with water and dried in Vacuo. Recrystallization from CH2Cl2/pentane afforded yellow-orange crystals of 5‚CH2Cl2, which were suitable for X-ray diffraction analysis (0.380 g, 62%), mp >200 °C dec. IR (KBr): ν (C ‚ ‚ C) + ν(C ‚ ‚ O) 1495 s. 1H NMR (CDCl3): δ 6.60-7.99 (m, 19 H, aromatic), 3.95 (d, 2H, CH2N, 4J(PH) ) 1.5 Hz), 2.85 (d, 6 H, Me2N, 4J(PH) ) 2.2 Hz). 31P{1H} NMR (THF/C6D6): δ 49.0 (s). Anal. Calcd for C29H27INOPPd‚CH2Cl2 (M ) 669.84 + 84.93): C, 47.74; H, 3.87; N, 1.86. Found: C, 47.85; H, 3.63; N, 1.83. Reaction of 5 with AuCl(PPh3). Solid AgBF4 (0.060 g, 0.306 mmol) was added to a stirred solution of AuCl(PPh3) (0.150 g, 0.303 mmol) in THF (30 mL) and the mixture stirred for 0.5 h. It was filtered, and the filtrate was added to complex 5 (0.230 g, 0.305 mmol) in THF (10 mL). After the mixture was stirred for 1 h, the solvent was removed under reduced pressure. A white solid was obtained which slowly decomposed to palladium metal. 31P{1H} NMR (CDCl3): δ 38.1 [d, 1 P, AuP, 3J(PP) ) 10.0 Hz], 63.4 [d, 1 P, PdP, 3J(PP) ) 10.0 Hz]. [(8-mq)Pd{Ph2PC(I) ‚ ‚ C( ‚ ‚ O)Ph}] (6). A solution of N-iodosuccinimide (0.125 g, 0.553 mmol) in THF (10 mL) was added to a

7180 Inorganic Chemistry, Vol. 35, No. 24, 1996

Andrieu et al.

Table 3. Crystal Data and Data Collection for 5 and 7‚CH2Cl2 mol formula fw cryst syst space group a, b, c (Å) β (deg) V (Å3); Z; Fcalcd (g cm-3) F (000); linear abs coeff (cm-1) radiation (graphite monochromator) no. of data collcd no. of unique data used, N R ) ∑(||Fo| - |Fc||)/∑|Fo| Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2 weighting scheme

C29H27NOPIPd 669.6 monoclinic P21/n 12.867 (3), 10.625 (3), 19.509 (6) 102.23 (2) 2606 (1); 4; 1.707 1320; 19.8 Mo KR (λ ) 0.710 73 Å) 3232 2798 [I > 6σ(I)] 0.029 0.034 1.000/[σ2(F) + 0.002941F2]

largest shift/esd, final cycle GOF ) [∑w(|Fo| - |Fc|)2/(Nobservn - Nparams)]1/2

2.04 (U12 C(22)) 0.82

solution of [(8-mq)Pd{Ph2PCH ‚ ‚ C( ‚ ‚ O)Ph}] (0.300 g, 0.543 mmol) in THF (20 mL). After being stirred for 8 h, the solvent was removed in Vacuo. The residue was washed several times with water (20 mL) and dried in Vacuo. The product was recrystallized from CH2Cl2/pentane, affording orange crystals of 6 (0.185 g, 55%), mp > 200 °C dec. IR (KBr): ν(C ‚ ‚ C) + ν(C ‚ ‚ O) 1501 s. 1H NMR (CDCl3): δ 8.30-7.25 (m, 21 H, aromatic), 3.10 (d, 2 H, CH2Pd, 3J(PH) ) 1.3 Hz). 31P{1H} NMR (CDCl3): δ 45.8 (s). Anal. Calcd for C30H23INOPPd (M ) 677.82): C, 53.16; H, 3.42; N, 2.07. Found: C, 53.15; H, 3.33; N, 2.09. [(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚O)Ph}Pd(I)(tmeda)] (7). To a mixture of 5‚CH2Cl2 (0.250 g, 0.331 mmol) and Pd(dba)2 (0.250 g, 0.364 mmol) was added toluene (30 mL) and the ligand tmeda (0.6 mL, 0.397 mmol). The dark red solution was stirred at 40-45 °C for 3 h, and a red suspension was obtained. The solvent was removed in Vacuo. The red residue was washed several times with Et2O (3 × 20 mL), dried in vacuo, redissolved in CH2Cl2 and filtered. Addition of pentane afforded red crystals which were suitable for X-ray diffraction analysis (0.296 g, 76%). IR (CH2Cl2): 1578 m, 1490 w, 1478 m, 1460 vs. 1H NMR (CD2Cl2): δ 9.0-6.6 (m, 19 H, aromatic), 4.20 (d, 1 H, CHAN, A part of an ABX spin system for CHAHBNPd, 2J(AB) ) 13.8 Hz, 4J(AX) < 2 Hz), 3.63 (dd, 1 H, CHBN, B part of an ABX spin system for CHAHBNPd, 2J(AB) ) 13.8 Hz, 4J(BX) ) 2.2 Hz), 3.00 (d, 3 H, MeAN of MeAMeBNPd, 4J(PH) ) 2.2 Hz), 2.70 (d, 3 H, MeBN of MeAMeBNPd, 4J(PH) ) 1.6 Hz), 2.61 (s, 3 H, MeAN of tmeda), 2.56 (s, 3 H, MeBN of tmeda), 2.35-1.86 (m, 4 H, N(CH2)2N of tmeda), 1.56 (s, 3 H, MeA′N of tmeda), 1.37 (s, 3 H, MeB′N of tmeda). 31P{1H} NMR (CD2Cl2): δ 56.3 (s). Anal. Calcd for C35H43IN3OPPd2‚CH2Cl2 (M ) 892.47 + 84.93): C, 44.24; H, 4.64; N, 4.30. Found: C, 44.33; H, 4.59; N, 4.32. [(dmba)Pd{Ph2PC ‚ ‚ C( ‚ ‚ O)Ph}Pd(I)(bipy)] (8). This complex was prepared by the same procedure as 7, starting from 5 (0.105 g, 0.157 mmol), Pd(dba)2 (0.089 g, 0.173 mmol) and solid bipy (0.027 g, 0.173 mmol); yield (0.095 g, 59%). IR (CH2Cl2): 1577 m, 1566 w, 1491 w, 1470 m, 1468 vs. 1H NMR (CD2Cl2): δ 9.6-6.5 (m, 27 H, aromatic), 4.07 (d, 1 H, CHAN, A part of an AB spin system for CHAHBNPd, 2J(AB) ) 13.9 Hz), 3.94 (d, 1 H, CHBN, B part of an AB spin system for CHAHBNPd, 2J(AB) ) 13.9 Hz), 3.00 (d, 3 H, MeAN of MeAMeBNPd, 4J(PH) ) 1.9 Hz), 2.90 (d, 3 H, MeBN of MeAMeBNPd, 4 J(PH) ) 1.8 Hz), 31P{1H} NMR (CD2Cl2): δ 55.3 (s). Anal. Calcd for C39H35IN3OPPd2‚CH2Cl2 (M ) 932.45): C, 50.24; H, 3.78; N, 4.51. Found: C, 49.97; H, 4.04; N, 4.42. Crystal Structure Determinations for Compounds 5 and 7‚CH2Cl2. The crystallographic data for both compounds are summarized in Table 3. Data were collected at room temperature (25 °C) on an Enraf Nonius CAD4 diffractometer (compound 5) and on a Siemens

C35H43IN3OPPd2‚CH2Cl2 977 monoclinic C2/c 35.906 (3), 13.565 (3), 15.775 (2) 95.099 (10) 7653 (2); 8; 1.655 3872; 19.5 Mo KR (λ ) 0.710 73 Å) 13665 4182 [I > 2σ(I)] 0.046 0.068 1.000/[σ2(F) + (0.0146P)2 + 71.33P] P ) (max(F02,O) + 2Fc2)/3 0.43 (U22 C(31)) 1.21

diffractometer equiped with an area charge coupled device detector (for 7‚CH2Cl2). The data reductions were performed respectively with the Enraf-Nonius SDP/VAX package15 and with the program SMART (Siemens Molecular Analysis Research Tool).16 Intensities were corrected for Lorentz and polarization effects and a morphology-based absorption correction was applied.17 Partial structures were solved using the Patterson methods of SHELXS18 (compound 5) or direct methods of phases determination of SHELXL19 (for 7‚CH2Cl2). Successive Fourier methods allowed the determination of the positions of the heavy atoms and refinement was done by full-matrix least-squares methods with anisotropic thermal parameters. All hydrogen atoms (excepting those of the CH2Cl2 molecule) were placed at their geometrically calculated positions and refined (for 5), with fixed C-H distances taking into account the hybridization of the carbon atom (for 7‚CH2Cl2). Their thermal parameters were fixed. The analytical scattering factors, corrected for the real and imaginary parts of anomalous dispersion, were taken from ref 20.

Acknowledgment. We thank the Centre National de la Recherche Scientifique (Paris) for financial support and the Ministe`re de l’Enseignement Supe´rieur et de la Recherche for a Ph.D. grant to J.A. We are grateful to the X-ray diffraction laboratory of the Siemens company in Karlsruhe, Germany, for the data collection and reduction of compound 7‚CH2Cl2. Supporting Information Available: Tables giving complete crystallographic experimental details (Table S-I), atomic coordinates of the non-hydrogen atoms (Tables S-II (5) and S-III (7‚CH2Cl2)), atomic coordinates of the hydrogen atoms (Tables S-IV (5) and S-V (7‚CH2Cl2)), anisotropic thermal parameters (Tables S-VI (5) and S-VII (7‚CH2Cl2)), and complete bond distances and angles (Tables S-VIII (5) and S-IX (7‚CH2Cl2)) (18 pages). Ordering information is given on any current masthead page. IC951507R (15) (a) Frenz, B. A. In Computing in Crystallography; Schenck, H., OlthofHazekamp, R., Van Koningveld, H.; Bassi, G. C. (eds), Delft University Press, Delft, 1978, pp 64-71. (b) Blessing, R. H. Cryst. ReV. 1987, 1, 3-58. (c) Blessing, R. H. J. Appl. Crystallogr. 1989, 22, 396-397. (16) Siemens Analytical Instruments Inc., Madison, WI. (17) De Titta, G. ABSORB: an absorption correction program for crystals enclosed in capillaries with trapped mother liquor. J. Appl. Crystallogr. 1985, 18, 75. (18) Sheldrick, G. M. SHELXS-86. Program for the solution of crystal structures. University of Go¨ttingen, Germany, 1986. (19) Sheldrick, G. M. SHELXL93. Program for Crystal Structure Refinement. University of Go¨ttingen, Germany, 1993. (20) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. 4.